1. Field of the Invention
The present invention relates to a liquid crystal display device and, particularly, to an active matrix liquid crystal display device of a high picture quality with wide viewing angle characteristics comprising high resistivity black matrix and with excellent light-shielding ability.
2. Description of the Prior Art
Display devices using a liquid crystal display device having reduced thickness and reduced weight and consuming decreased amounts of electric power have in recent years been widely used for personal computers, word processors and other information equipment.
Basically, the liquid crystal display device has a matrix made up of a number of electrodes arranged horizontally and vertically and a liquid crystal layer interposed between the horizontal electrodes and the vertical electrodes, and pixels are formed at the portions where two electrodes intersect each other to display a two-dimensional picture.
The liquid crystal display devices of this kind can be divided into those of a so-called passive matrix system which selects a predetermined pixel at a timing of a pulse applied to horizontal and vertical electrodes, and those of a so-called active matrix system in which a nonlinear element such as transistor or the like is provided in each pixel and a predetermined nonlinear element is selected.
In a liquid crystal display device of the active matrix system, non-linear elements (switching elements) are provided at positions corresponding to a plurality of pixel electrodes arranged in the form of a matrix. Theoretically, the liquid crystal in each pixel is driven at all times (duty ratio of 1.0). Compared with the so-called passive matrix system employing a time-division multiplex driving system, therefore, the active matrix system exhibits a good contrast and has been establishing a technique that is indispensable particularly in the color liquid crystal display devices. Thin-film transistors (TFTs) are representative examples of the switching elements.
In a conventional thin-film transistor liquid crystal display element, transparent electrodes are so formed as to face each other on the interface of two substrates to drive the liquid crystal layer.
That is, there has been employed a display system as represented by a so-called twisted nematic display system in which the direction of the electric field applied to the liquid crystals is almost perpendicular to the interface of the substrates owing to the employment of the above-mentioned electrode structure.
As a system in which the direction of an electric field applied to the liquid crystals is almost in parallel with the interface of the substrates, furthermore, there has been proposed a system (so-called an inplane electric field system) employing a pair of comb-toothed electrodes formed on the surface of at least one of the substrates for driving the liquid crystal layer as disclosed in, for example, Japanese Patent Publication No. 21907/1988 and Japanese Patent Laid-Open No. 36058/1995. The method of setting comb-toothed electrodes and the method of its fabrication have been proposed already by the present applicant in Japanese Patent Application No. 105862/1995.
In such an electrode structure, the major axes of molecules of the liquid crystal layer (hereinafter also referred to simply as liquid crystal molecules) are substantially in parallel with the surfaces of the substrates, and the liquid crystal molecules are suppressed from being erected in a direction perpendicular to the substrates. Therefore, the brightness changes little even when the viewing angle is changed; i.e., the so-called viewing angle dependence does not almost exist, and wide viewing angle characteristics are accomplished compared with those of the vertical electric field system.
In a conventional black mask interposed among the colored layers of various colors and constituting the substrate (color-filter substrate) of a color filter, furthermore, a pattern of thin film of metal chromium or low-reflection metal chromium has been formed. Or, a pattern of a thin layer of a photosensitive resin has been formed, dispersing a black coloring agent, or black carbon powder (chiefly graphite) therein, and adding various pigments thereto.
The colored layers of the color-filter substrate usually have a structure in which a protective layer PSV2 is formed on the colored layers FIL(R), FIL(G), FIL (B) of which the pattern regions are separated like a mosaic or vertical stripes for each of the pixels or colors.
When the colored layers are fabricated by using pigment-dispersed resin materials in the color filter used for the active matrix-type liquid crystal display devices, furthermore, the protective layer PSV2 is not often employed.
FIG. 13 is a schematic sectional view illustrating the constitution of a pixel that constitutes an inplane electric field-type liquid crystal display device and illustrating the turn-on operation, wherein reference numeral 1 denotes a TFT substrate, 1' denotes a color-filter substrate, 2 denotes a common electrode, 6 denotes an insulating film, 11 denotes pixel electrodes, 12 denotes a protective film, 17 denotes a black mask, and 18 denotes a color filter.
In FIG. 13, a liquid crystal layer is held between the TFT substrate 1 and the color filter substrate 1'. On the surfaces of the protective film 12 and of the color filter 18 are formed orientation films for establishing the initial orientation of liquid crystal molecules of the liquid crystal layer. Moreover, though not shown in FIG. 13, polarizer plates are installed on the outer surface sides of the TFT substrate 1 and the color filter substrate 1'.
In the above mentioned inplane electric field liquid crystal display device, the pixel electrodes 11 and the common electrode 2 are formed on one substrate (TFT substrate 1), and on the other substrate (color-filter substrate 1') is formed a color filter 18 that constitutes a color of a pixel demarcated by the black mask 17.
The black mask 17 demarcating the color filter prevents the reflection of external light and enhances the contrast by absorbing light from the neighboring pixels.